Tetrafluoroethylene polymers are of two types. One is non-melt-processible polymers where the melt viscosity is too high to process the polymers by ordinary melt-extrusion processes. Instead, the polymers are ordinarily sintered or paste extruded depending on the type polymer made. The other class is melt-processible tetrafluoroethylene copolymers having melt viscosities in the melt extrudable range.
Melt-processible tetrafluoroethylene (TFE) copolymer resins directly from the polymerizer and/or coagulator are referred to as fluff or powder. The fluff is normally humid heat treated and/or melt extruded to stabilize it, such as described in U.S. Pat. No. 3,085,083. There are applications such as rotocasting in which a free-flowing powder (herein called "granules") is preferable to melt-extruded pellets or where a high degree of purity of the resin is desired. Although rotolining and rotocoating processes differ in several technical respects from rotomolding, for the sake of convenience the term "rotocasting" is used herein to refer to all three generically unless otherwise indicated.
To facilitate handling of such granules, it is desirable to improve particle characteristics. Melt-processible copolymers that are coagulated from an aqueous dispersion and dried are friable, and form fines easily which give poor handling properties. It would be desirable to provide a melt-processible copolymer that is both stable and easily handled in a minimum of processing steps. It is particularly desirable to provide a copolymer that could be used both in conventional melt-fabrication processes and in rotocasting applications where particle characteristics are important.
It is also desirable to obtain resins that are thermally stable. A number of stabilization approaches are known in the art, most of which require melting the resins. Thus resins stabilized by these methods are generally available only as pellets--not (without tedious and expensive regrinding steps) as the free-flowing granules that are the basis of this invention.
Another desirable feature of such resins is that the granules should be low in metal contamination. If the granules have been melted in traditional thermoplastic processing equipment, contamination occurs inevitably when the corrosive tetrafluoroethylene copolymer melts come in contact with the interior metal surfaces of thermoplastic processing equipment, even when corrosion-resistant alloys are used. Copolymers having low levels of metal contamination are particularily desirable for applications in the semiconductor industry.